favor placeholders over existentials when choosing SCC representatives

... even when the existential has the least RegionVid.

universal regions (of root universe) > placeholders > existentials

The previous behavior, that chooses the minimal RegionVid index, naturally prefers universal regions over others
because they always have the least RegionVids, but there was no guranteed ordering between placeholders and existentials.

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -95,9 +95,11 @@ pub struct RegionInferenceContext<'tcx> {
     /// visible from this index.
     scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
 
-    /// Contains a "representative" from each SCC. This will be the
-    /// minimal RegionVid belonging to that universe. It is used as a
-    /// kind of hacky way to manage checking outlives relationships,
+    /// Contains the "representative" region of each SCC.
+    /// It is defined as the one with the minimal RegionVid, favoring
+    /// free regions, then placeholders, then existential regions.
+    ///
+    /// It is a hacky way to manage checking regions for equality,
     /// since we can 'canonicalize' each region to the representative
     /// of its SCC and be sure that -- if they have the same repr --
     /// they *must* be equal (though not having the same repr does not
@@ -481,22 +483,28 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         scc_universes
     }
 
-    /// For each SCC, we compute a unique `RegionVid` (in fact, the
-    /// minimal one that belongs to the SCC). See
+    /// For each SCC, we compute a unique `RegionVid`. See the
     /// `scc_representatives` field of `RegionInferenceContext` for
     /// more details.
     fn compute_scc_representatives(
         constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
         definitions: &IndexSlice<RegionVid, RegionDefinition<'tcx>>,
     ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
         let num_sccs = constraints_scc.num_sccs();
-        let next_region_vid = definitions.next_index();
-        let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
-
-        for region_vid in definitions.indices() {
-            let scc = constraints_scc.scc(region_vid);
-            let prev_min = scc_representatives[scc];
-            scc_representatives[scc] = region_vid.min(prev_min);
+        let mut scc_representatives = IndexVec::from_elem_n(RegionVid::MAX, num_sccs);
+
+        // Iterate over all RegionVids *in-order* and pick the least RegionVid as the
+        // representative of its SCC. This naturally prefers free regions over others.
+        for (vid, def) in definitions.iter_enumerated() {
+            let repr = &mut scc_representatives[constraints_scc.scc(vid)];
+            if *repr == ty::RegionVid::MAX {
+                *repr = vid;
+            } else if matches!(def.origin, NllRegionVariableOrigin::Placeholder(_))
+                && matches!(definitions[*repr].origin, NllRegionVariableOrigin::Existential { .. })
+            {
+                // Pick placeholders over existentials even if they have a greater RegionVid.
+                *repr = vid;
+            }
         }
 
         scc_representatives